Angiogenesis (the formation of new blood vessels) is an important process in tumor growth, wound healing, retinopathies, and rheumatoid arthritis. We are investigating aspects of angiogenesis in an in vitro model utilizing isolated human and bovine endothelial cells and a mixture of basement membrane matrix proteins (Matrigel) or collagen gel. Normally endothelial cells are quiescent in vivo; but in response to an angiogenic factor, activated endothelial cells will breakdown their underlying basement membrane, migrate into the interstitium, proliferate, and finally differentiate into a new blood vessel. We found that quiescent endothelial cells (contact inhibition or serum starvation) were not able to organize into a capillary-like network in vitro, while cells released from contact inhibition or feed serum- containing media regained the ability to organize. There was a correlation between the ability to organize on Matrigel and the expression of cell surface receptors for the basement membrane proteins, laminin and collagen IV. These studies should prove useful in studying some early events involved in angiogenesis. Angiogenic factors attract leukocytes and fibroblast in an area, while stimulating the formation of new blood vessels. We tested whether fibroblast produced some factor that altered the ability of endothelial cells to organize into new vessels. We found that fibroblast- conditioned media contained a cellular attractant and enhanced the extent of capillary-like formations on Matrigel or within a collagen gel. The ability of fibroblast to influence angiogenesis may be an important component of wound-healing. We found that TGF-beta caused differentiated endothelial cells on Matrigel or on collagen 1 gels to undergo apoptosis. TGF-beta may be important for the final stages of wound healing when the area forms scar tissue and becomes avascular.